Ability to change value set on model, based on suggestion

I've added a new, optional suggestionKey attribute to the directive that allows me to specify what data from the suggestion object should actually be used on the model, rather than using the entire suggestion object.

Use case: We have a bloodhound engine that returns full objects, but only a certain key is being used as the display.  We'd like that same key to be used for the model after the update is performed.  But, we're listening to the typeahead:selected and typeahead:autocompleted to do other work with the full suggestion object.

If you'd like me to update the base README with this new attribute, I'd be happy to do so.